The board of directors of any irrigation district in the State of Nebraska which has issued valid interest-bearing bonds that are outstanding and unpaid, may take up and pay off any such bonds whenever the same can be brought about by lawful means, by the issue and sale or the issue and exchange therefor of the bonds of such irrigation district; but bonds so to be issued shall not exceed the amount lawfully owing and unpaid upon the bond or bonds so sought to be taken up and paid. Bonds so issued shall not bear interest greater in rate or amount per annum than the bonds so sought to be taken up and paid.